Langerhans cells (LC) constitute a subset of dendritic cells (DC) that initiate innate immune responses in skin, and are able to instruct the acquired T cell response by the presentation of peptide antigen via MHC molecules. A unique aspect of LC is the co-expression of CD1a and the C-type lectin langerin. Through study of the skin lesions of leprosy, we discovered a unique function of LC: the ability to present microbial antigens by a langerin-dependent, CD1a-restricted pathway. In addition to their instructive role on acquired T cell responses, we demonstrated that LC have a direct innate function by their ability to induce an antiviral response. During the past three funding periods, in addition to investigating the role of LC in host defense against microbial infection, our studies have contributed to CD1 immunobiology, by defining the structure of lipid-containing CD1-restricted T cell antigens and mechanisms of CD1 antigen presentation, identified mechanisms that regulate CD1 expression in skin, and provided the first evidence for a direct T cell mediated antimicrobial activity. Based on our progress and preliminary data, we hypothesize that LC instruct the acquired T cell response by the langerin-dependent, CD1a-restricted presentation of lipoglycoprotein antigens to T cells and by a direct innate vitamin D-dependent antimicrobial pathway. We propose studies to advance our knowledge of LC and CD1 immunobiology, in relationship to host defense in skin, by investigating leprosy, an infection of skin caused by the obligate intracellular pathogen Mycobacterium leprae (mLEP). Our specific aims are to 1) elucidate the structure of antigens presented by LC via CD1a to T cells, 2) determine the role of langerin in CD1a-restricted antigen presentation by LC to T cells; and 3) investigate the antimicrobial function of CD1- restricted T cells and LC in skin. The studies we propose are intended to provide a comprehensive and in depth view of LC and CD1-restricted T cell function in relation to a model of human skin disease, with the goal of understanding mechanisms of host defense to microbial pathogens in skin.